The present invention relates to a magnetic field correcting method, magnetic field generating apparatus, and magnetic resonance imaging apparatus, and more particularly to a method of correcting an inhomogeneity error of a static magnetic field generated by a pair of magnets that are supported by yokes so that the magnets face each other across a space, an apparatus for generating a static magnetic field of which an inhomogeneity error is corrected, and a magnetic resonance imaging apparatus comprising such a magnetic field generating apparatus.
In a conventional apparatus for generating a homogeneous static magnetic field in a space by a pair of magnets facing each other, two pairs of circular loop coils disposed symmetrically with respect to a center of the static magnetic field space are employed to cancel (correct) a quadratic term component of the static magnetic field by quadratic term components of magnetic fields generated by the circular loop coil pairs. The magnetic fields generated by the two pairs of circular loop coils are made to have opposite polarities to reduce the effect of their zero-th order term components on the static magnetic field.
In such a conventional method, since the polarities of the magnetic fields generated by the two pairs of circular loop coils are opposite to each other, the quadratic term component of the static magnetic field is corrected by the difference between the quadratic term components of the magnetic fields from the circular loop coil pairs. Accordingly, efficiency of the correction is poor considering the power supplied to the two pairs of circular loop coils.
It is therefore an object of the present invention to provide a method of efficiently correcting a quadratic term component of a static magnetic field, an apparatus for generating a static magnetic field of which a quadratic term component is efficiently corrected, and a magnetic resonance imaging apparatus comprising such a magnetic field generating apparatus.
(1) The present invention, in accordance with one aspect thereof for solving the aforementioned problem, is a magnetic field correcting method for correcting an inhomogeneity error of a static magnetic field generated in a space by a pair of magnets that are supported by yokes so that the magnets face each other across said space, characterized in comprising: correcting a quadratic term component of said static magnetic field by a quadratic term component of a magnetic field generated in said space by a pair of circular loop coils disposed in said space symmetrically with respect to a center of said space; and compensating a zero-th order term component of the magnetic field in said space from said pair of circular loop coils by a zero-th order term component of a magnetic field generated in said space by a coil wound around at least one of said yokes.
(2) The present invention, in accordance with another aspect thereof for solving the aforementioned problem, is a magnetic field generating apparatus characterized in comprising: magnetic field generating means having a pair of magnets that are supported by yokes so that the magnets face each other across a space, for generating a static magnetic field in said space; correcting means for correcting a quadratic term component of said static magnetic field by a quadratic term component of a magnetic field generated in said space by a pair of circular loop coils disposed in said space symmetrically with respect to a center of said space; and compensating means for compensating a zero-th order term component of the magnetic field in said space from said pair of circular loop coils by a zero-th order term component of a magnetic field generated in said space by a coil wound around at least one of said yokes.
(3) The present invention, in accordance with still another aspect thereof for solving the aforementioned problem, is a magnetic resonance imaging apparatus characterized in comprising: magnetic field generating means having a pair of magnets that are supported by yokes so that the magnets face each other across a space, for generating a static magnetic field in said space; correcting means for correcting a quadratic term component of said static magnetic field by a quadratic term component of a magnetic field generated in said space by a pair of circular loop coils disposed in said space symmetrically with respect to a center of said space; compensating means for compensating a zero-th order term component of the magnetic field in said space from said pair of circular loop coils by a zero-th order term component of a magnetic field generated in said space by a coil wound around at least one of said yokes; signal acquiring means for acquiring magnetic resonance signals generated by spins in said space; and image producing means for producing an image based on said acquired magnetic resonance signals.
In the invention of the aforementioned aspects, when an inhomogeneity error of a static magnetic field generated by a pair of magnets that are supported by yokes so that the magnets face each other across a space is to be corrected, a quadratic term component of the static magnetic field is corrected by a quadratic term component of a magnetic field generated by a pair of circular loop coils disposed symmetrically with respect to a center of the static magnetic field space, and a zero-th order term component of the magnetic field from the pair of circular loop coils is compensated by a zero-th order term component of a magnetic field generated by a coil wound around at least one of the yokes.
Since the quadratic term component of the magnetic field generated by the coil wound around the yoke is significantly smaller than the quadratic term component of the magnetic field generated by the pair of circular loop coils, the correction effect by the pair of circular loop coils on the quadratic term component is not diminished. Thus, the quadratic term component of the static magnetic field is efficiently corrected. At the same time, the effect of the zero-th order term component of the magnetic field from the pair of circular loop coils on the static magnetic field is appropriately eliminated.
Preferably, sensing means for sensing the magnetic field in said space from said pair of circular loop coils, and control means for controlling said compensating means based on said sensed magnetic field are included so that the zero-th order term component may be properly corrected.
Preferably, a pair of gradient coils for imparting a gradient to the static magnetic field in said space is provided on pole surfaces of said pair of magnets, and said pair of circular loop coils is provided along outer peripheries of said pair of gradient coils so that increase of the distance between the magnets due to the provision of the pair of loop coils may be reduced.
Preferably, said yokes comprise at least one vertical yoke and a pair of horizontal yokes facing each other horizontally extending from ends of said vertical yoke so that the static magnetic field space may be appropriately formed.
Preferably, said pair of magnets is provided on opposing surfaces of said pair of horizontal yokes when a vertical magnetic field is to be generated.
Preferably, the magnets of said pair each have a pole piece so that homogeneity of the magnetic field may be improved.
Preferably, said at least one vertical yoke comprises a pair of yokes symmetric with respect to the center of said space so that the horizontal yokes may be doubly supported.
Preferably, said coil is wound around said vertical yoke so that the magnetic field for correcting the zero-th order term component may be appropriately generated.
Therefore, the present invention can provide a method of efficiently correcting a quadratic term component of a static magnetic field, an apparatus for generating a static magnetic field of which a quadratic term component is efficiently corrected, and a magnetic resonance imaging apparatus comprising such a magnetic field generating apparatus.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.